2001 Honda Civic Heater does not blow floor air ducts

Hello, When the heater is turned on in this car it does not blow from the air ducts that are aimed at your feet / floor. I am able to switch to the air ducts that blow towards your face and switch to defrost mode and heat comes out as expected. But its very unpleasant to be blasted by hot air in your face while your feet are cold. My question is: what can be causing this and how do I go about repairing this problem? Thanks !